The importance of lithium ion batteries, nickel hydride batteries and other secondary batteries as power sources to be carried on vehicles or power sources for personal computers and portable terminals has grown in recent years. In particular, lightweight lithium ion batteries that make it possible to obtain a high energy density are expected to be advantageously used as high-output power sources to be carried on vehicles.
A battery structure including a wound electrode body having a spirally wound structure is known as a typical configuration of a lithium ion battery. The wound electrode body is constituted by sheet-like electrodes (sheet-like positive electrode and sheet-like negative electrode) and a sheet-like separator and manufactured by laminating the sheets and winding the laminate. The sheet-like electrodes for the lithium ion battery have a configuration in which an electrode active material layer including an electrode active material is formed on one or both surfaces of a sheet-like substrate made from a metal. The sheet-like electrode is provided with an uncoated portion (a portion where the electrode active material layer has not been formed and the surface of the sheet-like substrate is exposed) in part thereof, and an electrode lead (current-collecting foil or the like) is attached to the uncoated portion.
For example, a method by which an electrode active material layer is coated as a continuous stripe pattern with a predetermined spacing in the longitudinal direction of a sheet-like substrate is known as a coating method providing an uncoated portion on the sheet-like electrode. A coating means such as a die coater can be advantageously used for such a coating method. For example, a method is used by which a die is disposed in a convening path of the sheet-like substrate to coat the electrode active material layer. The conventional methods for measuring the thickness (height) of the coated film are disclosed in Patent Documents 1 and 2.    Patent Document 1: Japanese Patent Application Publication No. 2002-257506    Patent Document 2: Japanese Patent Application Publication No. 2006-71625
However, the problem associated with the sheet-like electrode coated by the above-described die-coater or the like is that the thickness of the electrode active material layer can easily become uneven at the boundary of the electrode active material layer and the uncoated portion. Thus, when the electrode active material layer is coated on the substrate, a coating liquid drip occurs at the coating end portion of the electrode active material layer, and even if the die shape or viscosity of the coating liquid is adjusted, the thickness in the transverse direction is still uneven and the thickens of the coating portion of the electrode active material layer tends to be less than that in the coating center portion. In the coating end portion of the electrode active material layer where the thickness has decreased, the coating weight per unit surface area (coating amount) decreases and therefore unsuitable coated portions that do not have the required coating weight can be obtained. The capacity of a lithium ion secondary battery is increases by efficiently packing the electrode active material in as large an amount as possible into the limited inner space of the battery. Therefore, it is important that the occurrence of such unsuitable coated portions that do not have the required coating weight be prevented and the coating width (yield of electrode active material) be managed.